This invention relates to a non-retrievable plug for sealing the bore of hollow cylindrical or tubular members, particularly conductor pipe employed with offshore platforms, and to a method for sinking such conductor pipes.
A typical offshore platform has a plurality of conductor pipes running from the top of the platform to the bottom thereof. After the platform is set in place, the conductor pipes are driven into the sea bottom and wells drilled from the platform through the pipes. To give added buoyancy to the platform as it is moved and set in place, the end of each conductor pipe is sealed with a plug to prevent the ingress of water. As an offshore platform may be 1,000 or more feet from top to bottom, the plug in the conductor pipe must be capable of withstanding large hydrostatic pressures when the platform is installed on the sea bottom.
Several different prior art approaches have been taken in plug design and construction. One typical prior art plug comprises a steel cup retained within a cylindrical housing welded to the end of a conductor pipe of the same diameter. The steel cup is retained within the housing by means of a molded elastomeric member which has a portion of the cup retrieving cable spirally wrapped within. To retrieve the cup from the conductor pipe an upward force is applied to the free end of the cable at the top of the pipe, whereupon the cable molded in the elastomeric member progressively rips the elastomeric member apart, freeing the steel cup to move upwardly in the conductor pipe. Such a plug is easy to use, but it is difficult to control the molding process and the accurate placement of the cable within the elastomeric member to ensure proper performance. Furthermore, this type of plug leaves a residue of elastomeric material within the conductor pipe, which may, if large enough, interfere with subsequent drilling operations through the pipe. Another approach to conductor pipe plugs is disclosed in U.S. Pat. No. 4,178,967, issued on Dec. 18, 1979 to Steven G. Streich and assigned to the assignee of the present application. The patent discloses a retrievable, reusable conductor pipe plug which is mechanically locked into place in a housing welded to the bottom of the conductor pipe until it is retracted and withdrawn from the conductor pipe by application of upward force on a cable. While this type of plug is reusable, manufacturing costs are relatively high and failure or jamming of the retraction mechanism is always possible, which events would require the destruction of the plug and removal of the resulting debris, a laborious and difficult procedure. In addition, unless the plug incorporates a device to equalize pressure on both sides of the plug, it is necessary to fill the conductor pipe with fluid prior to removal to prevent rapid and possibly damaging upward movement of the plug due to the large hydrostatic forces acting upon the plug bottom. Finally, certain parts of the plug must be replaced prior to every re-use.
In using the prior art plugs, the step of removing the plug comprises an additional operation to the procedure of driving the conductor pipe and sinking the well therethrough, requiring additional time on the job as well as care in removing the plug.